Besides looking silly, I think anyone who knows what's up and what's down on a piano will call the bluff after 5 seconds. And the best way to reveal that you don't know something would be to openly fake it.

Besides looking silly, I think anyone who knows what's up and what's down on a piano will call the bluff after 5 seconds.

Of course they will. But this is something that you would do in front of people who don't know anything about playing a piano, or who otherwise aren't paying attention to what's being played People don't tend to question the "music" being played by someone as long as it sounds reasonably nice.